The long-term objective of this proposal is to demonstrate that the heart is a self-renewing organ and, therefore, cardiac aging is determined by the progressive depletion of functionally competent cardiac stem cells (CSCs). The major hypothesis to be tested is that the reduction in the pool size of resident CSCs is dependent on the accumulation of G-rich single stranded fragments, downregulation of telomere related proteins, formation of chromatin anaphase bridges and telomeric shortening. These forms of DNA damage together with the expression of gene products blocking the cell cycle define aged CSCs. Senescent CSCs acquire a permanent and irreversible biological status, which consists of growth arrest i n G0-G1. T heir distribution and localization in the heart may influence CSC aging. CSC aging could occur first in anatomical regions exposed to high hemodynamic stress, such as the base and mid-portion of the left ventricle, and later in the atria, apex and right ventricle, which are subjected to significantly lower levels of hemodynamic loads. CSC aging negatively affects the cell turnover of the heart resulting in the accumulation of old myocytes and a chronic decline in young more efficient and powerful cells. This is because CSCs divide symmetrically or asymmetrically in the old heart, giving rise to lineage committed cells without preserving the CSC pool size. These hypotheses will be tested during physiological aging in rats, and in mice with spontaneous mutation and inactivation of the c-kit receptor. The rat model will be employed to provide a detailed characterization of the relationship between the age-dependent changes in the biological behavior of c-kit(POS) CSCs and heart aging. This possibility will be confirmed by ablation and replenishment interventions in irradiated rats. The W/W v mouse with a non-functional c-kit receptor will be studied in order to define the actual role of c-kit(POS) CSCs in the evolution of myocardial aging. If the notion of heart aging we have suggested has some validity, replenishment of functionally competent c-kit(POS) CSCs in W/W v mice can be expected to reverse premature aging of the heart in these animals. Conversely, the imposition of an overload by aortic banding should accelerate the development of a decompensated myopathy dictated by limitations in the generation of new myocytes and vascular structures by the non-responding c-kit (POS) CSCs. This should mimic the senescent failing heart. Again, treatment with intact c-kit(POS) CSCs might positively interfere with the myopathy. In conclusion, loss of CSC function by forced quiescence may condition aging and senescence of the mammalian heart.